rennen
to run
To move very quickly on foot; to run fast.
Die Kinder rennen um die Wette zum Spielplatz.
The children run to the playground in a race.

to run
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to run
To move very quickly on foot; to run fast.
Die Kinder rennen um die Wette zum Spielplatz.
The children run to the playground in a race.

As a verb of fast movement, 'rennen' (to run) always forms the perfect tense with the auxiliary verb 'sein' (to be). A common mistake is using 'haben' (to have).
Ich bin zum Bahnhof gerannt, um den Zug noch zu erwischen.
I ran to the station to still catch the train.
To emphasize the high speed, 'rennen' (to run) is often combined with certain expressions. Examples are 'um sein Leben rennen' (to run for one's life), 'um die Wette rennen' (to race), or 'so schnell man kann rennen' (to run as fast as one can).
Als der Hund bellte, ist das Kind schnell weggerannt.
When the dog barked, the child quickly ran away.
When 'rennen' (to rush) is used in the sense of 'sich beeilen' (to hurry), it is typical of colloquial language. In more formal contexts, you would use other verbs.
Ich muss noch schnell zur Bank rennen, bevor sie schließt.
I still have to quickly rush to the bank before it closes.
The expression 'gegen eine Wand rennen' (to run into a wall) is a fixed idiom. It means that you meet insurmountable resistance with a plan or idea and have no success.
Mit seiner Idee ist er bei seinem Chef leider gegen eine Wand gerannt.
Unfortunately, he ran into a wall with his idea when he brought it to his boss.
to go
'gehen' and 'rennen' both describe movement on foot, but they are opposites in terms of speed. 'gehen' is normal and calm. 'rennen' is very fast and fits when someone is doing sports, is in a hurry, or urgently wants to get somewhere. Learners may confuse the words because both happen without a vehicle and are done with the legs. The practical difference is simple: someone who has time walks; someone who has to be very fast runs.

Max rennt zur Haltestelle, weil der Bus gleich kommt.
Max runs to the bus stop because the bus is about to arrive.
Im Park gehen wir langsam am See entlang.
In the park, we walk slowly along the lake.